Printing liquids are used in many applications such as painting, gravure and press printing, xerographic printing using liquid toners, and inkjet printing. Printing liquids typically comprise a colorant in a liquid medium or vehicle. Examples include paints, liquid toners, and inks. The vehicle may be organic-based (solvent-based) or water-based (aqueous-based). The colorant may be dye or pigment, or a combination thereof. The printing liquid may further include additional ingredients based on the particular application.
Inkjet printing is a non-impact printing process in which droplets of ink are deposited on print medium, such as paper, transparency film, or textiles. Essentially, inkjet printing involves the ejection of fine droplets of ink onto print medium in response to electrical signals generated by a microprocessor. Inkjet printers offer low cost, high quality printing with relatively noise-free operation. As such, inkjet printers have become a popular alternative to other types of printers.
There are two basic means currently available for achieving ink droplet ejection in inkjet printing: thermally and piezoelectrically. In piezoelectric inkjet printing, the ink droplets are ejected due to the vibrations of piezoelectric crystals, again, in response to electrical signals generated by the microprocessor. The ejection of ink droplets in a particular order forms alphanumeric characters, area fills, and other patterns on the print medium.
A conventional thermal color inkjet printer comprises a plurality of resistor elements arranged in a particular pattern in a printhead. The resistor elements are located in a chamber that is provided with an opening for inkjet ink to enter from a reservoir. Together, the printhead and the reservoir, comprise an inkjet pen. The printhead also includes an orifice plate having a plurality of orifices through which inkjet ink is expelled toward a print medium. Each resistor element is connected by a conductive trace to a microprocessor, where current-carrying signals cause one or more selected resistor elements to heat up. The heat creates bubbles of vaporized ink in the chambers, which force the ink to expel through the orifices toward the print medium. The properly sequenced ejection of inkjet ink from each orifice causes characters or other images to be printed on the print medium as the printhead is moved across the print medium. The inkjet printers produce high quality printing and are both compact and affordable. In addition, since only the ink strikes the paper, the inkjet printer is fast and quiet.
To produce high quality images, the inkjet ink must be compatible with the inkjet pen and the print medium, as well as the rest of the printing system. The properties of an optimal inkjet ink include, among other things, good crusting resistance, good stability, low color-to-color and black-to-color bleed, and rapid dry time. In addition, the inkjet ink must be capable of passing through the inkjet orifice without clogging the orifice or puddling on the orifice plate. The inkjet ink should also permit rapid cleanup of the machine components with minimal effort while being environmentally friendly.
While the overall print quality of currently produced inkjet inks is generally high, puddling of these inks may still occur on the orifice plate of the printhead. Puddling occurs when the ink that is ejected through the orifices does not reach the print medium. Instead, the inkjet ink collects on an outer surface of the orifice plate or puddles adjacent to the edge of the orifice. The extent of the puddling varies from a few small drops of ink to the formation of large puddles on large portions of the orifice plate. Large puddles partially or completely block the orifices and cause missing nozzles, false low decap values, or changes in the trajectory of the ink drops. The change in trajectory results in the ink drop not hitting its targeted pixel center, which creates printing errors on the media and reduces the quality of the printed image.
Various solutions to address the problem of puddling have been proposed. Some solutions propose modifying the printhead or pen to reduce puddling, while other solutions modify the inkjet ink composition. For example, some approaches include the coating of hydrophobic material applied to the printhead to reduce its wettability and, thereby, reduce puddling; combination of pen architecture and inks; and modifications to the inkjet ink composition.
Other proposed modifications to the inkjet inks include adding anionic and nonionic surfactants to the inkjet ink. One example of surfactants used in inkjet inks are fluorinated compounds which may be nonionic, anionic, cationic, or amphoteric. Examples of such fluorinated compounds include those commercially available from E. I. du Pont de Nemours and Company (Wilmington, Del.) as Zonyl®, and from 3M Company (Minneapolis, Minn.) as Fluorad®. These surfactants are commonly classified as telomer which refers to a mixture of perfluorinated oligomers of carbon chain length from C6 to C18 possessing a reactive functional group.
It would be desirable to reduce puddling of inkjet inks by using additives that are effective in low amounts and do not negatively affect other properties of the inkjet inks. In addition, it would be desirable to use additives that are effective in many types of ink vehicles.